The present invention relates to a drive system for driving valves and auxiliary machinery by a crankshaft of an overhead camshaft engine for a motor vehicle, and more particularly to an arrangement of timing belts in the system.
Japanese Utility Model Laid Open No. 56-113127 discloses an automotive engine wherein an oil pump is mounted on a crankshaft of the engine. A crankshaft pulley engaged with a timing belt for driving a camshaft is secured to the crankshaft at a position outside the oil pump, and an auxiliary machinery driving pulley engaged with a driving belt for driving an auxiliary machine is securely mounted on the crankshaft outside the crankshaft pulley. In the system, since the oil pump and pulleys are all disposed in different vertical planes with respect to the crankshaft, the outermost pulley projects considerably out of the crankcase so that the length of the engine increases. Additionally, the system is arranged to directly drive a camshaft pulley from the crankshaft pulley through the timing belt. Accordingly the gear ratio for the camshaft is determined by the ratio between the diameters of both of the pulleys. In order to provide an appropriate gear ratio, the diameter of the camshaft pulley becomes much larger than that of the crankshaft pulley. Thus, durability and operativity of the timing belt are not good.